1//===------ SimplifyInstructions.cpp - Remove redundant instructions ------===//
2//
3//                     The LLVM Compiler Infrastructure
4//
5// This file is distributed under the University of Illinois Open Source
6// License. See LICENSE.TXT for details.
7//
8//===----------------------------------------------------------------------===//
9//
10// This is a utility pass used for testing the InstructionSimplify analysis.
11// The analysis is applied to every instruction, and if it simplifies then the
12// instruction is replaced by the simplification.  If you are looking for a pass
13// that performs serious instruction folding, use the instcombine pass instead.
14//
15//===----------------------------------------------------------------------===//
16
17#include "llvm/Transforms/Scalar.h"
18#include "llvm/ADT/DepthFirstIterator.h"
19#include "llvm/ADT/SmallPtrSet.h"
20#include "llvm/ADT/Statistic.h"
21#include "llvm/Analysis/InstructionSimplify.h"
22#include "llvm/IR/DataLayout.h"
23#include "llvm/IR/Dominators.h"
24#include "llvm/IR/Function.h"
25#include "llvm/IR/Type.h"
26#include "llvm/Pass.h"
27#include "llvm/Target/TargetLibraryInfo.h"
28#include "llvm/Transforms/Utils/Local.h"
29using namespace llvm;
30
31#define DEBUG_TYPE "instsimplify"
32
33STATISTIC(NumSimplified, "Number of redundant instructions removed");
34
35namespace {
36  struct InstSimplifier : public FunctionPass {
37    static char ID; // Pass identification, replacement for typeid
38    InstSimplifier() : FunctionPass(ID) {
39      initializeInstSimplifierPass(*PassRegistry::getPassRegistry());
40    }
41
42    void getAnalysisUsage(AnalysisUsage &AU) const override {
43      AU.setPreservesCFG();
44      AU.addRequired<TargetLibraryInfo>();
45    }
46
47    /// runOnFunction - Remove instructions that simplify.
48    bool runOnFunction(Function &F) override {
49      const DominatorTreeWrapperPass *DTWP =
50          getAnalysisIfAvailable<DominatorTreeWrapperPass>();
51      const DominatorTree *DT = DTWP ? &DTWP->getDomTree() : nullptr;
52      DataLayoutPass *DLP = getAnalysisIfAvailable<DataLayoutPass>();
53      const DataLayout *DL = DLP ? &DLP->getDataLayout() : nullptr;
54      const TargetLibraryInfo *TLI = &getAnalysis<TargetLibraryInfo>();
55      SmallPtrSet<const Instruction*, 8> S1, S2, *ToSimplify = &S1, *Next = &S2;
56      bool Changed = false;
57
58      do {
59        for (BasicBlock *BB : depth_first(&F.getEntryBlock()))
60          // Here be subtlety: the iterator must be incremented before the loop
61          // body (not sure why), so a range-for loop won't work here.
62          for (BasicBlock::iterator BI = BB->begin(), BE = BB->end(); BI != BE;) {
63            Instruction *I = BI++;
64            // The first time through the loop ToSimplify is empty and we try to
65            // simplify all instructions.  On later iterations ToSimplify is not
66            // empty and we only bother simplifying instructions that are in it.
67            if (!ToSimplify->empty() && !ToSimplify->count(I))
68              continue;
69            // Don't waste time simplifying unused instructions.
70            if (!I->use_empty())
71              if (Value *V = SimplifyInstruction(I, DL, TLI, DT)) {
72                // Mark all uses for resimplification next time round the loop.
73                for (User *U : I->users())
74                  Next->insert(cast<Instruction>(U));
75                I->replaceAllUsesWith(V);
76                ++NumSimplified;
77                Changed = true;
78              }
79            bool res = RecursivelyDeleteTriviallyDeadInstructions(I, TLI);
80            if (res)  {
81              // RecursivelyDeleteTriviallyDeadInstruction can remove
82              // more than one instruction, so simply incrementing the
83              // iterator does not work. When instructions get deleted
84              // re-iterate instead.
85              BI = BB->begin(); BE = BB->end();
86              Changed |= res;
87            }
88          }
89
90        // Place the list of instructions to simplify on the next loop iteration
91        // into ToSimplify.
92        std::swap(ToSimplify, Next);
93        Next->clear();
94      } while (!ToSimplify->empty());
95
96      return Changed;
97    }
98  };
99}
100
101char InstSimplifier::ID = 0;
102INITIALIZE_PASS_BEGIN(InstSimplifier, "instsimplify",
103                      "Remove redundant instructions", false, false)
104INITIALIZE_PASS_DEPENDENCY(TargetLibraryInfo)
105INITIALIZE_PASS_END(InstSimplifier, "instsimplify",
106                    "Remove redundant instructions", false, false)
107char &llvm::InstructionSimplifierID = InstSimplifier::ID;
108
109// Public interface to the simplify instructions pass.
110FunctionPass *llvm::createInstructionSimplifierPass() {
111  return new InstSimplifier();
112}
113